That's a fair point, but it falls flat with me personally the same way arguments about gun control and the potential to use guns in revolutions against autocratic regimes fall flat with me: it just seems incredibly unlikely in this day and age, at least in developed countries.

Like, the US once rounded up Japanese people and put them in camps. Using your same reasoning, one could argue that the government collecting any data on race/ethnicity anywhere -- like, say, on the census -- is a slippery slope to interning racial minorities again. And yet, almost nobody really cares in practice, because almost nobody thinks there's a realistic chance of that happening again.

Governments have also, at times, discriminated against people for their sexuality, or gender, so should the government never be allowed to gather any information on those attributes?

>it just seems incredibly unlikely in this day and age, at least in developed countries.

This opinion wouldn't be quite as naive if there was some magical boundary that keeps digital information solely in the hands of benevolent rulers in developed countries, but there's not.

>Governments have also, at times, discriminated against people for their sexuality, or gender, so should the government never be allowed to gather any information on those attributes?

That's somewhat like how some gun rights advocates say it's futile to ban guns because you can just kill people with knives, so why don't we ban knives too?

We're talking about virtually total information awareness which is far more powerful, problematic, and discriminate than knowledge of superficial attributes such as race and gender.

https://en.m.wikipedia.org/wiki/Total_Information_Awareness

To be fair, none of that involved selling ads.

It involved the mass collection of personal information (what Google does in order to sell ads), which was then weaponized and used for purposes other than originally implied